Circuit breaker with thermal tripping means



July 14, 1964 A. R. CELLERINI CIRCUIT BREAKER WITH THERMAL TRIPPING MEANS Filed Feb. l5, 1960 5 Sheets-Sheet 1 INVENTOR Albert R. Cellerini ATTORNEY July 14, 1964 A. R. cELLERfNl 3,141,081

CIRCUIET BREAKER WITH THERMAL TRIPPING MEANS Filed Feb. 15, 1960 5 Sheets-Sheet 2 July 14, 1964 A. R. cELLr-:RlNx 3,141,081

CIRCUIT BREAKER WITH THERMAL TRIPPING MEANS Filed Feb. 15; 19Go 5 sheets-sheet 3 United States Patent() 3,141,081 CIRCUIT EREAKER WITH THERMAL 'IRIPPHNG MEANS Albert R. Cellerini, Beaver, Pa., assigner to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 15, 1960, Ser. No. 8,818 3 Claims. (Cl. 20G-122) This invention relates to circuit breakers, and more particularly, to trip devices for effecting automatic tripping of circuit breakers.

An object of the invention is to provide an improved trip device for eilecting automatic opening of a circuit breaker.

Another object of the invention is to provide a circuit breaker having a trip device embodying a conducting member that can withstand heavy short circuits Without incurring serious damage.

A further object of the invention is to provide a circuit breaker having a trip device comprising a bimetallic member and a substantially straight conducting member for heating the bimetallic member.

Another object of the invention is to provide a circuit breaker embodying a trip device with a conductor having a portion of reduced cross sectional area for more eiiective heating of the bimetallic member.

A further object of the invention is to provide a circuit breaker embodying a trip device with improved means for heating the bimetallic member by conducting current through the bimetallic member.

Another object of the invention is to provide a circuit breaker embodying a trip device wherein an induced current is utilized in order to effect heating ofthe bimetallic member.

Another object of the invention is to provide a circuit breaker embodying a trip device with adjustable means heated by an induced current to effect tripping of the breaker.

A more general object of the invention is to provide a circuit breaker with an improved trip device that can be manufactured at a reduced cost.

The novel features that are considered characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to structure and operation, together with additional objects and advantages thereof, will be best understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

FIGURE l is a side view, in section, of a circuit breaker embodying the principal features of the invention;

FIG. 2 is an enlarged View, partly in side elevation and partly in section of a trip device utilized in the circuit breaker illustrated in FIG. 1;

FIG. 3 is an end elevational View,` on an enlarged scale of part of the trip device the cover being broken away to show the device more clearly;

FIG. 4 is a side elevational View of the conductor and the bimetallic member of the trip device shown in FIG. 2;

FIG. 5 is a View looking in the direction of line V-V of FIG. 4;

FIG. 6 is a side elevational view of another embodiment of a conductor and a bimetallic member for the trip device;

FIG. 7 is a side elevational view of another embodiment of a conductor and a bimetallic member for the trip device.

FIG. 8 is a top View of the conductor and the bimetallic member shown in FIG. 7;

FIG. 9 is a side elevational View of a different embodiment of parts of the trip device.

3,141,081 Patented July 14, 1964 FIG. 1G is a side elevational view of still another ernbodiment of parts of the trip device;

FIG. ll is a view taken along line XI-XI of FIG. l0; and Y FIG. l2 is a View similar to FIG. 11 of still another embodiment of parts of the trip device.

Referring to FIGURE l of the drawings, the circuit breaker shown therein comprises, generally, a base 11 and a removable cover 13, both of which may be molded from a suitable insulating material. The breaker is of the threepole type, each pole being provided with terminal structures indicated generally at 15 and 17 which are mounted at opposite ends of the base 11. These terminal structures may be of the type specifically described and claimed in the copending application of Albert R. Cellerini and Louis N. Ricci Serial No. 8,819, led February 15, 1960, and assigned to the assignee of the instant application.

The circuit breaker includes, for each pole unit, a stationary main contact 21, a stationary arcing Contact 22, a movable main contact 23, a movable arcing contact 24 and an arc extinguisher indicated generally at 25. A common operating mechanism indicated generally at 27 is provided for simultaneous actuation of the movable contacts to opened and closed positions. A trip device indicated generally at 29 serves to effect automatic opening of the circuit breaker contacts in response to predetermined overload conditions in the circuit of any pole unit of the circuit breaker. The terminal structure 15, for each pole unit, is mounted at the outer end of a terminal conducting strip 31 which extends into the housing and rigidly supports the stationary contacts 21 and 22.

The movable contacts Z3 and 24, for each pole unit, are mounted on a rigid contact arm 33 supported on a switch arm 35 which is secured to a tie bar 37 that extends across all of the pole units of the circuit breaker. The tie bar 37 supports the switch arms 35 for the several pole units of the breaker for unitary movement to the open and closed positions. Each of the contact arms 33 is connected by means of flexible conductors 39 to an intermediate conducting strip 41. The conducting strips 41 are each connected to a trip unit conducting strip, or heating member, which is indicated generally at 43, by a bolt 42 which connects the conductors 43 and 41 to the base 11 of the breaker. Each of the trip unit conducting strips 43 passes through the trip unit 29 and is supported at its outer end on a conducting block 45. A terminal structure 17, for each pole unit, is mounted at the outer end of each one of the trip unit conductors 43.

The circuit through each pole unit of the breaker extends from the terminal structure 15 through the conducting strip 31, stationary contacts 21 and 22, movable contacts 23 and 24, movable contact arm 33, tiexible conductors 39, intermediate conducting strip 41, trip unit conducting strip 43, and to the load terminal structure 17.

The operating mechanism 27 (FIG. l) is disposed in the center compartment of the housing and is supported by a pair of spaced frame members 53 (only one being shown) secured to the base 11. The operating mechanism comprises a pivoted forked operating lever 55, a toggle comprising toggle links 57 and 59, two overcenter springs 61 (only one being shown) and a pivoted releasable cradle 63 controlled by the trip device 29. An arcuate insulating shield 67, for substantially closing an opening 69 in the cover 13, is mounted on the outer end of the operating lever 55 and has an integral handle portion 71 extending out through the opening 69 to permit manual operation of the breaker.

The toggle links 57 and 59 are pivotally connected together by a knee pivot pin 73. The toggle link 57 is pivotally connected to the cradle 63 by a pin 75 and the toggle link 59 is pivotally connected to the switch arm a 35, ot the center pole unit, by a pin 77. The overcenter springs 61 are connected under tension between the knee pivot pin 73 of the toggle 57, 59 and the outer end of the operating lever 55. The circuit breaker is manually operated to the open position by movement of the handle 71 (FIG. 1) in a clockwise direction to the ott position, which movement actuates the overcenter springs 61 to cause collapse of the toggle 57, 59 and opening movement of the switch arms 3S for all of the pole units of the circuit breaker in a manner well known in the art.

The circuit breaker is manually closed by reverse movement of the handle 71 from the ot to the on position, which movement causes the springs 61 to move overcenter and straighten the toggle 57, S9 thereby moving the switch arms 35, for all of the pole units, to the closed position.

The trip device 29, (FIGS. 2 and 3), includes a molded insulating trip bar 75 which is common to all of the pole units of the breaker. The trip bar 75 is pivotally supported by means of pins 77 (only one being shown in FIGS. 2 and 3) which are supported in a bracket 79 (FIG. 2) that is secured to the base 11 by one of the screws 42. The bracket 79 also supports an insulating base 31 for the trip device. A cover $3 (FIG. 2), of insulating material, cooperates with the base 81 to enclose the parts of the trip device.

A latch S (shown in broken lines in FIG. 2), on the trip bar 75, normally engages a latch portion S6 of a latch mechanism indicated generally at 93. The latch mechanism 93 engages and releasably restrains the cradle 63 (FIG. l) to releasably hold the operating mechanism in the closed position.

A tripping electromagnet, indicated generally at 95 (FIGS. 2 and 3), includes a xed, laminated, U-shaped magnet yoke 917, the laminations being secured together by rivets not shown. The magnet yoke 97 is supported by means of rivets 99 (FIG. 3) on a bracket 101 (FIG. 2) which is supported on the insulating base 81 by the rivets 99. A movable armature 103 is pivotally supported by means of a knife-edge pivot 105 (FIG. 3) in a V-shaped notch in the xed yoke 97. A rod 107 is pivotally connected to the armature 103, and it has a head 109 for engaging and operating the trip bar 75 upon energization of the magnet yoke 97. Two springs 111 are connected under tension between an end of the armature 103 and a portion of the fixed magnet yoke 97. The springs 111 bias the armature 103 counterclockwise to the unattracted position shown in FIG. 3. The tripping electromagnet 95 is energized by current owing in the conductor 43 which extends between the legs of the U-shaped magnet yoke 97.

The circuit breaker is tripped open instantaneously by operation of the tripping electromagnet 95. When the magnet is energized by an overload current of, for instance, 500% of normal rated current, the armature 103 is attracted pulling the rod 107 down, whereupon the head 109, of the rod 107, engages the trip bar 75 causing the trip bar to rotate clockwise as seen in FIG. 2 to disengage the latch 85 from the latch mechanism $3 which, in turn, releases the cradle 63 (FIG. 1).

Upon release of the cradle 63, the springs 61 act to rotate the cradle in a counterclockwise direction about its pivot 64, thus causing collapse of the toggle 57, 59 and movement or" the switch arms 35 and movable contacts for all of the pole units to the open position.

An adjusting mechanism, indicated generally at 11S in FIGS. 2 and 3, is provided so that the minimum overload current required to instantaneously trip the breaker can be selected within a predetermined range. This is accomplished by adjusting the position ot' the armature 103 relative to the magnet yoke 97 to thereby vary the magnetic air gap. The adjusting mechanism is specifically described and claimed in the patent to Glenn R. Thomas and Robert H. Flick, entitled Circuit Breaker, Patent No. 3,073,925, issued January 15, 1963 and assigned to the assignee of the instant application. For this reason, only l a brief description of the adjusting mechanism is given herein.

The adjusting mechanism 115 comprises a cam member 117 having a cam surface 119 which is engaged by one end 121 of a lever 123. An adjustable screw 125 is threaded through the other end ot the lever 123, and it engages the armature 103. The lever 123 is pivotally supported intermediate its ends on a pivot pin 127 (FIG. 3) which is supported by a bracket 129 that is supported in the trip device by the rivets 99. The cam member 117 is supported on a bent-over portion 131 (FIG. 2) of the bracket 101. A rod 133 extends from the cam member 117. An adjusting knob 135 is provided at the outer end ofthe rod 133 and it is supported against lateral movement by a bent-over portion 137 (FIG. 2) of the bracket 129.

Upon rotation of the adjusting knob 135, the cam member 117 is rotated and the cam-following end 121, of the lever 123, follows the cam surface 119 of the cam member 117, to cause the lever 123 to pivot about the pin 127. As the lever 123 pivots, the armature 103, which is biased against the screw by the springs 111, is pivoted about its pivot 105 to vary the magnetic air gap, thereby varying the minimum overload current which is required to instantaneously trip the breaker.

It is necessary to reset and relatch the breaker mechanism, following an automatic opening operation, before the contacts can be closed. Resetting and relatching is eiected by moving the handle 71 (FIG. 1) clockwise as far as it will go. During this movement, a projection 141 on the operating lever engages a shoulder on the cradle 63 and moves the latter in a clockwise direction. Near the end of this movement, the free or latching end of the cradle 63 is reengaged with the latch mechanism 93 in a well-known manner. The breaker contacts are then closed in the previously described manner by movement of the handle to the on position.

The instantaneous tripping operation, upon the occurrence of overload currents above a predetermined value, is desirable in order to protect the current carrying parts from damage due to sever overheating. As is well known in the art, it is also desirable to have a time delay before the breaker trips automatically upon the occurrence of overload currents below the predetermined value so that the breaker will not trip open if the ovenload is of short duration. The breaker can be calibrated to trip open after a time delay in response to overloads of, for example, between 100% and 300% of the normal rated current, and to trip open instantaneously, by operation of the tripping electromagnet, upon the occurrence of overloads greater than 300% of the normal rated current.

In accordance with this invention, improved means are provided for effecting automatic tripping of the breaker with a time delay. The time-delay tripping operation is effected, in circuit breakers presently in use, by means of a looped conductor or heating member having a straight bimetal attached to one leg of the loop. The heat generated in the conductor by overload currents is conducted to the bimetal causing the biunetal to bend to effect an automatic tripping operation after a time delay. Upon the occurrence of heavy short circuits, the repelling forces of the looped conductor operate to expand the loop, often bending the conductor out of shape and damaging the bimetal. Thus, the thermal calibration of a circuit breaker under such conditions is often destroyed.

The present invention eliminates this loop effect by provding a trip device embodying thermal tripping means having a substantially straight heating member or conducting member and a generally L-shaped bimetallic member having one leg attached to the substantially straight heating member or conducting member. In addition to the elimination of the loop eiect, `the step in manufacturing of bending the conducting strip is eliminated by the provision of a substantially straight heating member or conducting member in the trip device. As illustrated in FIGS. 2, 4 and 5, the conducting member or heating member 43 comprises a number of conducting laminations 145 having outer conducting plates 147. An opening 149 (FIG. 4) is provided in the conducting member 43 for receiving the mounting bolt 42 (FIGS. 1 and 2) which connects the conducting member 43 to the conductor 41. An opening 151 (FIG. 4) is provided in the yconducting member 43 for receiving a mounting bolt 153 (FIG. 1) whichconnects the terminal structure 17 to the conducting member 43. As seen in FIG. 4, one of the lower laminations 147 extends further to the right than the rest of the conducting member 43 providing a recess in the member 43 for receiving the terminal structure 17 (FIG. 1). The conducting member 43, however, is substantially straight in that there are no loops therein which would expand upon the occurrence of heavy overload currents flowing therethrough. A generally L-shaped bimetallic member 155 is secured, at one leg thereof, to the conducting member 43 by means of two rivets 157.

The high expansion side of the bimetallic member 155 is to the left as Viewed in FIG. 2. Upon the occurrence of an overload current below the predetermined value, the heat generated in the conducting member 43 is conducted to the bimetallic member 155, causing the bimetallic member to bend to the right to engage an adjustable screw 159 in the trip bar 75 to cause the trip bar 75 to rotate clockwise to disengage the latch 85 from the latch mechanism 93 which, in turn, releases the cradle 63 (FIG. l). Upon release of the cradle 63, the breaker trips open in the same manner hereinbefore described. In addition to being more dependable in operation, the trip device, with the substantially straight conducting member 43, is more easily manufactured and assembled. Moreover, this trip device can be manufactured at a reduced cost.

Another embodiment of the invention is shown in FIG. 6. A straight conducting member or heating member 161 is provided with a portion 163 having a reduced cross sectional area. The conducting member has less height at 163. One leg of a generally L-shaped bimetallic member 165 is riveted to the conducting member 161 at the reduced portion 163. It is well known that the resistance of a conductor increases as the cross sectional area decreases. A11 increased amount of heat, therefore, is generated by the increased resistance at the reduced portion 163. It can be understood that this embodiment of the invention provides adequate heating capacity for the generally L-shaped bimetallic member 165 even though only a small portion of the bimetallic member contacts the conducting member 161 and the major portion of the bimetallic member is not adjacent the conducting member to receive heat radiated therefrom.

Another embodiment of the invention is shown in FIGS. 7 and 8. In this embodiment, a straight conducting vmember or heating member 169 is provided with a portion 171 having a reduced cross sectional area. The conducting member has less width at 171. One Yleg of a generally L-shaped bimetallic member 173 is riveted to the conducting member 169 at the reduced portion 171. It is well known that corners and edges will heat more readily than the rest of a conductor upon the passage of current through the conductor. The additional corners and edges provided at the reduced portion 171 of the conducting member 169, therefore, provide the conducting member with additional capacity to heat the bimetallic member 173. It can be seen in FIG. 8, that the bimetal 173 is not positioned directly over all of the additional corners and edges made available by the provision of the reduced portion 171. Heat generated at these corners and edges, however, is conducted through the conductor to the birnetal. It can be understood that when additional thermal sensitivity is required, a birnetal and conductor can be provided wherein the bimetal is positioned directly over the additional corners and edges made available by the provision of a reduced portion of the conductor.

Another embodiment of the invention is shown in FIG. 9. Two straight conducting members or heating 6 members 177 and 179 overlap each other and sandwich between them one leg of a generally L-shaped bimetallic member 181. The conducting members 177 and 179, and the bimetallic member 181, are riveted together as shown. It can be understood that the bimetallic member 181 will be heated not only by the heat conducted from thefconducting members 177 and 179,- but also by the current which passes through the bimetallic member. The current passes through the bimetallic member 181 in a direction normalL to the planes of the Hat faces. The bimetallic member, therefore, can carry a considerable amount of current without being unduly heated because of the large cross sectional area that carries the current.

Another embodiment of the invention is shown in FIGS. 10 and ll. As shown therein, a substantially straight conducting member or heating member 183, which is formed of two laminations 184 and 186, passes, through a solid iron core member 185 which forms a one turn magnetic circuit about the conducting member.

The lower lamination 186 (FIG. 10) extends further to the right thm the rest of the conducting member 183 providing a recess in the member 183 for receiving the terminal structure 17 (FIG. 1). The conducting member 183, however, is substantially straight in that there are no loops therein which would expand upon` the occurrence of heavy overload currents flowing therethrough. A screw 187 is screwed up through a tapped opening in the core member 185 and it extends into an opening 189 in the conducting member 183 to hold the conducting member in place. The opening 189 is larger than the screw 187 to permit the conducting member 183 to move or float in the assembly. This provides flexibility for easier assembly and alignment of parts. A generally L- shaped bimetallic member 191 is Welded or brazed at one leg thereof to the core member 185.

Core losses appear in the form of heat in the core member 185 when an alternating current flows through the conducting member 183. This heat comprises the heat generated by magnetic hysteresis losses and also the heat generated by eddy currents which are induced in the core member 185. The heat generated in the core member is conducted to the bimetllic member 191 to cause thermal reaction of the bimetal and thermal tripping of the breakerk in the same manner hereinbefore described. It can be seen that the conducting member 183 is substantially straight to enable it to better withstand heavy overload currents. Another advantage is that this embodiment can be used in breakers of high ratings because the thermal action of the bimetal is not caused by heat generated in the conducting member, which member can be made large to run cool in the breaker and to withstand heavier overload currents. The bimetallic member 191 can withstand heavier overload currents because it does not contact the main conducting member 183.

Another embodiment of the invention is shown in FIG. 12. This embodiment is similar to that shown in FIGS. 10 and l1 except that an adjustable core member indicated generally at 193 is provided in place of the core member 185 of the other embodiment. The core member 193 is of magnetic material, preferably iron, and it comprises a lower portionk 195 and an upper portion 197. Two non-magnetic shims 199 are provided at each side between the lower and upper portions of the core member 193, and the parts are secured together by two screws 201. The insertion of the shims 199 between the parts of the core member serves to increase the resistance of the magnetic circuit to provide less heating effect per unit of current flowing through the conducting member 183. The bimetal thermal reaction, and, therefore, the thermal tripping characteristics of the breaker can be adjusted by varying the number of shims 199 to be placed between the parts of the core member. An advantage of this embodiment is that the same trip device can be used with a number of breakers of different ratings.

The invention provides an improved trip device embodying a substantially straight conducting member. A generally L-shaped bimetallic member is attached at one leg thereof to the conducting member, the other leg being generally perpendicular to the conducting member. The heatiing capacity of the conducting member can be increased by providing a reduced portion to carry the Same amount of current. The capacity of the conducting member to heat the bimetallic member can be increased also by providing an increased number of corners and edges on the conducting member. The amount of thermal reaction can be increased by passing the circuit breaker current through the bimetallic member ina direction normal to the plane of the greatest cross sectional area of the bimetallic member.

Improved means are also provided for heating the bimetal by means of an induced current. A one-turn magnetic circuit is formed around the conducting member by a core member. An alternating current passing through the conducting member induces eddy currents in the core member to heat the core member. Hysteresis losses account for additional heating of the core member 185. The heat from the core member is conducted to a bimetal which is attached thereto, and the thermal reaction of the bimetal is utilized to effect tripping of the breaker. The magnetic core member can be made in two parts with means for inserting non-magnetic shims between the parts to vary the bimetal thermal reaction per amount of current so that the same trip device can be used with breakers of different ratings.

While the invention has been disclosed in accordance with the provisions of the patent statutes, it is to be understood that various changes and modifications in the structural details and arrangement of parts of the various embodiments thereof may be made without departing from some of the essential features of the invention. It is desired, therefore, that the language of the appended claims be given as reasonably broad an interpretation as the prior art permits.

I claim as my invention:

1. A circuit breaker comprising, separable contacts and means releasable to effect automatic separation of said contacts, a trip device for effecting release of said releasable means, said trip device comprising a core member comprising a generally U-shaped structure of magnetic material and a top part of magnetic material, means removably mounting said top part on the free ends of said U-shaped structure whereby said top part and said U- shaped structure encompass an opening, a conductor connected in electrical series with said contacts and passing through said opening, an elongated bimetallic member supported at one end thereof on said top part in a heat conducting relationship with said top part and the other end thereof extending away from said core member in the direction of the extension of the legs of said U-shaped structure, said top part being adapted to be heated by hysteresis and eddy current losses therein when said conductor is energized by an alternating current, said heat in said top part being conducted to said bimetallic member to heat said bimetallic member, and said bimetallic member when heated a predetermined amount bending to effect release of said releasable means.

2. A circuit breaker comprising, separable contacts and means releasable to effect automatic separation of said contacts, a trip device for effecting release of said releasable meaus, said trip device comprising a core member,

said core member comprising a generally U-shaped structure of magnetic material and a top part of magnetic material, a separate member of non-magnetic material disposed at each of the free ends of said U-shaped structure, securing means securing said top part to the free ends of said U-shaped structure and securing said nonmagnetic members between said top part and said U- shaped structure to form a core member encompassing an opening, a conductor connected in electrical series with said contacts and passing through said opening, an elongated bimetallic member supported at one end on said top part in a heat conducting relationship with said top part and extending away from said core member in the direction of the extension of the legs of said U-shaped structure, said top part being adapted to be heated by hysteresis and eddy current losses therein when said conductor is energized by an alternating current, the heat in said top part being conducted to said bimetallic member to heat said bimetallic member, said bimetallic member when heated a predetermined amount bending to effect release of said releasable means, and said securing means being removable to permit replacement of said members of non-magnetic material with other members of non-magnetic material whereby said core member can be adjusted to thereby vary the tripping characteristics of said circuit breaker.

3. A circuit breaker comprising, a stationary contact and a movable contact cooperable therewith to open and close said breaker, means releasable to trip open said contacts, a conductor connected in electrical series with said contacts, a core member comprising a generally U- shaped structure of magnetic material and a top part of magnetic material, a separate member of non-magnetic material disposed at each of the free ends of said U- shaped structure, securing means securing said top part to the free ends of said U-shaped structure and securing said non-magnetic members between said top part and said U-shaped structure to form a core member encompassing an opening, said conductor passing through said opening, said top part being adapted to be heated by hysteresis and eddy current losses therein when an alternating current flows through said conductor, a bimetallic member supported on said top part in a heat conducting relationship with said top part, said bimetallic member upon the occurrence of a predetermined Overload current in said conductor being heated and bending to effect release of said releasable means after a time delay, and said securing means being removable to permit replacement of said members of non-magnetic material with other members of non-magnetic material whereby said core member can be adjusted to thereby vary the tripping characteristics of said circuit breaker.

References Cited in the file of this patent UNITED STATES PATENTS 2,050,284 Dorfman Aug. 11, 1936 2,340,957 Grissinger Feb. 8, 1944 2,342,945 Krieger Feb. 29, 1944 2,476,022 Bennett July 12, 1949 2,629,796 Kern Feb. 24, 1953 2,824,922 Jennings Feb. 25, 1958 2,939,929 Hobson June 7, 1960 FOREIGN PATENTS 151,064 switzerland Feb. 1, 1932 

1. A CIRCUIT BREAKER COMPRISING, SEPARABLE CONTACTS AND MEANS RELEASABLE TO EFFECT AUTOMATIC SEPARATION OF SAID CONTACTS, A TRIP DEVICE FOR EFFECTING RELEASE OF SAID RELEASABLE MEANS, SAID TRIP DEVICE COMPRISING A CORE MEMBER COMPRISING A GENERALLY U-SHAPED STRUCTURE OF MAGNETIC MATERIAL AND A TOP PART OF MAGNETIC MATERIAL, MEANS REMOVABLY MOUNTING SAID TOP PART ON THE FREE ENDS OF SAID U-SHAPED STRUCTURE WHEREBY SAID TOP PART AND SAID USHAPED STRUCTURE ENCOMPASS AN OPENING, A CONDUCTOR CONNECTED IN ELECTRICAL SERIES WITH SAID CONTACTS AND PASSING THROUGH SAID OPENING, AN ELONGATED BIMETALLIC MEMBER SUPPORTED AT ONE END THEREOF ON SAID TOP PART IN A HEAT CONDUCTING RELATIONSHIP WITH SAID TOP PART AND THE OTHER END THEREOF EXTENDING AWAY FROM SAID CORE MEMBER IN THE DIRECTION OF THE EXTENSION OF THE LEGS OF SAID U-SHAPED STRUCTURE, SAID TOP PART BEING ADAPTED TO BE HEATED BY HYSTERESIS AND EDDY CURRENT LOSSES THEREIN WHEN SAID CONDUCTOR IS ENERGIZED BY AN ALTERNATING CURRENT, SAID HEAT IN SAID TOP PART BEING CONDUCTED TO SAID BIMETALLIC MEMBER TO HEAT SAID BIMETALLIC MEMBER, AND SAID BIMETALLIC MEMBER WHEN HEATED A PREDETERMINED AMOUNT BENDING TO EFFECT RELEASE OF SAID RELEASABLE MEANS. 